1. Field of the Invention
The present invention relates to a power converter that generates predefined AC voltage using voltage of an AC power source and a DC power source. More particularly, the present invention relates to instantaneous voltage-drop compensation devices and an uninterruptible power source device that can supply stabilized voltage to a load, even upon occurrence of fluctuation of voltage of an AC power source and power failure in the AC power source.
2. Related Art
FIG. 15 is a diagram for explaining a power converter of a continuous-inverter power feeding scheme disclosed in Japanese Patent Application Publication No. H7-337036 (also referred to herein as “Patent document 1”). The power converter converts temporarily the voltage of an AC power source to DC voltage, converts the DC voltage again to AC voltage, and supplies the voltage to a load. In the figure, the reference symbol 1 is a single-phase AC power source, 2 is a capacitor, 3 is a converter circuit, 4 is an inverter circuit, 5 is a filter circuit, and 6 is a load.
One end of the AC power source 1 is connected to an AC input terminal of the converter circuit 3. The AC input terminal of the converter circuit 3 is connected to one end of the AC power source 1. In the converter circuit 3, one end of a reactor L is connected to the AC input terminal. The other end of the reactor L is connected to a connection midpoint of a switching element series circuit, in which switching elements Qp, Qn are connected in series. A capacitor series circuit in which capacitors Cp, Cn are connected in series, is connected to both ends of the switching element series circuit. The connection midpoint of the capacitor series circuit is connected to the other end of the AC power source 1. The converter circuit 3 turns the switching elements Qp, Qn on and off, to rectify the voltage of the AC power source 1, and charge the capacitors Cp, Cn to a predefined voltage. The capacitors Cp, Cn thus charged to a predefined voltage form a DC power source.
The capacitor 2 is connected between the AC input terminal of the converter circuit 3 and the connection midpoint of the capacitor series circuit. The inverter circuit 4 comprises series-connected switching elements Q1, Q2. The inverter circuit 4 is connected to a DC output terminal of the converter circuit 3. The inverter circuit 4 turns on and off the switching elements Q1, Q2, to convert, to AC voltage, the voltage of the DC power source that comprises the capacitors Cp, Cn.
The filter circuit 5 is configured through connection in series of a reactor Lf1 and a capacitor Cf1. One end of the filter circuit 5 is connected to the connection midpoint of the switching elements Q1, Q2. The other end of the filter circuit 5 is connected to the connection midpoint of the capacitor series circuit. The filter circuit 5 removes a high-frequency component from the AC voltage that is outputted from the inverter circuit 4.
One end of the load 6 is connected to the connection point of the reactor Lf1 and the capacitor Cf1. The other end of the load 6 is connected to the other end of the AC power source 1. The AC voltage that is supplied from the inverter circuit 4 is outputted, via the filter circuit 5, to the load 6.
FIG. 16 is a diagram for explaining a power converter of a continuous commercial-power feeding scheme disclosed in Japanese Patent Application Publication No. H11-178216 (also referred to herein as “Patent document 2”). In the figure, a switch 7 and the secondary winding of a transformer 8 are connected in series between an AC power source 1 and a load. The respective connection relationships between a converter circuit 3, an inverter circuit 4, a filter circuit 5 and a capacitor 2 are identical to those of the embodiment in FIG. 15. An AC input terminal of the converter circuit 3 is connected to one end of the primary winding of the transformer 8. The connection midpoint of the capacitor series circuit is connected to the other end of the AC power source 1 and is connected to the other end of the primary winding of the transformer 8. The connection point of the reactor Lf1 and the capacitor Cf1 is connected to one end of the load 6.
The power converter supplies ordinarily voltage of the AC power source to the load. When the voltage of the AC power source 1 drops, the converter circuit 3 turns on and off the switching elements Qp, Qn, to generate thereby compensation voltage for compensating the voltage drop from the DC voltage at which the capacitor series circuit is charged.
The compensation voltage is superimposed on the voltage of the AC power source 1, via the transformer 8. The voltage resulting from superimposing the compensation voltage on the voltage of the AC power source 1 is supplied to the load 6. Charging of the capacitor series circuit is carried out in this case by the inverter circuit 4.
The switch 7 is opened when the AC power source 1 fails. The inverter circuit 4 turns on and off the switching elements Q1, Q2, to convert the DC voltage of the capacitor series circuit to AC voltage, and supply the voltage to the load 6.
In the power converter illustrated in FIG. 15, however, AC-DC voltage conversion by the converter circuit 3 and DC-AC voltage conversion by the inverter circuit 4 are required until AC voltage is supplied from the AC power source 1 to the load 6. The current that flows through the power converter passes at least once through each switching element of the converter circuit 3 and the inverter circuit 4. That is, the current flowing in the power converter passes through switching elements at least two or more times. Accordingly, respective conduction losses derived from passage of current through the switching elements occur at both the converter circuit 3 and of the inverter circuit 4.
The on and off operations of the switching elements Q1 to Q4 in the converter circuit 3 and the inverter circuit 4 are performed on the basis of the voltage of the DC power source that comprises the capacitors Cp and Cn. Accordingly, switching loss occurs when each element is turned on or turned off.
The power loss, including conduction loss and switching loss, in the switching elements is therefore substantial in the technology disclosed in Patent document 1. A problem arises herein in that the conversion efficiency of the power converter drops when power loss in the switching elements is large.
In the power converter illustrated in FIG. 16, the transformer 8 is required in order to compensate for the voltage drop of the AC power source 1. The size of the transformer 8 is large, since the latter must function at a commercial frequency. In the power converter illustrated in FIG. 16, moreover, the operations of the converter circuit 3 and the inverter circuit 4 must be switched in order to supply predefined AC voltage to the load 6 when a power failure occurs in the AC power source 1.
A problem arises therefore, in the technology disclosed in Patent document 2, in that a large commercial transformer is required, which translates into a power converter of large size. A further problem is the occurrence of disturbances in the AC output voltage as a result of switching over between the operations of the converter circuit 3 and the inverter circuit 4.